Compressed air systems are used to provide energy for driving a variety of devices in a variety of applications. One such application is a railroad locomotive where compressed air is used to power locomotive air brakes and pneumatic control systems.
A typical compressed air system will include a motor-driven compressor to maintain the air pressure in a reservoir within a desired range of pressures. The compressor is cycled on and off in response to a measurement of pressure in the reservoir. A bypass valve is connected to the outlet of the compressor to selectively vent the compressor to atmosphere for running the compressor in an unloaded mode. The unloaded mode is used when the compressor motor is first energized in order to reduce the starting current drawn by the motor. After the compressor/motor have come up to speed, the bypass valve is closed to place the compressor in the loaded mode for supplying compressed air to the reservoir. After a desired pressure is achieved in the reservoir, the compressor is allowed to run in the unloaded mode for a short period, such as 30 seconds, in order to cool down the compressor and motor components. At the end of the cool down period, the motor is de-energized and the system stands ready to be re-started when the reservoir pressure drops below the low-pressure set point.
On occasion, the compressor/motor will fail to achieve a desired speed within a predetermined time period after the motor is energized. This may be due to a variety of problems, including mechanical failures in the motor or compressor, electrical failures in the motor, power supply or connections, or an improperly positioned bypass valve that leaves the compressor in the loaded mode during start-up. Regardless of the cause of the problem, the failure of the compressor to achieve a desired speed within a predetermined time period will result in the motor being tripped in order to prevent excessive heat buildup in the motor, and a system fault will be logged. With repeated failures to start, the compressed air system will be declared out of service in an effort to protect the induction motor from thermal breakdown, thereby adversely impacting the availability of the locomotive for use.